The Hunter License... I believe. However there are certain rules. That are revealed later.

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The Ten Hunter Commandments:

1) Hunters must hunt something
2) All Hunters require a minimum level of martial proficiency. Being able to use nen is the minimum level necessary.
3) No matter what may happen, one who has become a Hunter will never have their title revoked. However, no matter the circumstances, a Hunter license will never be issued a second time.
4) Hunters may not hunt their fellow Hunters, except for those who have committed atrocities.
5) Hunters who achieve Extraordinary accomplishments in certain fields will receive a star.
6) Veteran Hunters who have received a star, become superior officers and have had their students receive stars will receive two stars.
7) Hunters with two stars who achieve extraordinary accomplishments in multiple fields will receive three stars.
8) If the Hunter in charge does not receive a minimum level of support, then they will have their post removed. In this case, the minimum level is support from the majority of his fellow Hunters. If the seat of chairman becomes open, an election for the next chairman must happen immediately. The vice-chairman is to be left in charge until the new chairman is decided.
9) The chairman decides how the new Hunters are chosen. But in order to make a great change in the existing method, he must not receive support from the majority.
10) Everything not written here is to be decided by the chairman, vice-chairman, and their advisers. The chairman has the right to choose who becomes the vice-chairman and advisers.

For someone to be a pro hunter they have to pass the Hunter Exam and learn Nen.
Of course there is the rank system (1 star, 2 star, 3 star), but it is not a necessity to be called pro hunter. However, those who achieve it have high status. As an example, Ging is a double star (2 star) hunter.

For someone to be a pro hunter they have to pass the Hunter Exam and learn Nen.
Of course there is the rank system (1 star, 2 star, 3 star), but it is not a necessity to be called pro hunter. However, those who achieve it have high status. As an example, Ging is a double star (2 star) hunter.

I was so sure he was a triple.

Edit:

nnnn;

Yep I just re-watched the episode and it didn't say anything about Ging being a triple.
Just that he is as good as a triple.